HETEROGENEITY OF CHLORIDE CHANNELS IN THE APICAL MEMBRANE OF ISOLATEDMITOCHONDRIA-RICH CELLS FROM TOAD SKIN

The isolated epithelium of toad skin was disintegrated into single cel ls by treatment with collagenase and trypsine. Chloride channels of ce ll-attached and excised inside-out apical membrane-patches of mitochon dria-rich cells were studied by the patch-clamp technique. The major p opulation of Cl- channels constituted small 7-pS linear channels in sy mmetrical solutions (125 mM Cl-). In cell-attached and inside-out patc hes the single channel i/V-relationship could be described by electrod iffusion of Cl- with a Goldmann-Hodgkin-Katz permeability of, P-Cl=1.2 x10(-14) -2.6x10(-14) cm(3) . s(-1). The channel exhibited voltage-ind ependent activity and could be activated by cAMP. This channel is a li kely candidate for mediating the well known cAMP-induced transepitheli al Cl- conductance of the amphibian skin epithelium. Another populatio n of Cl- channels exhibited large, highly variable conductances (upper limit conductances, 150-550 pS) and could be activated by membrane de polarization. A group of intermediate-sized Cl--channels included: (a) channels (mean conductance, 30 pS) with linear or slightly outwardly rectifying i/V-relationships and activity occur-ring in distinct ''bur sts,'' (b) channels (conductance-range, 10-27 pS) with marked depolari zation-induced activity, and (c) channels with unresolvable kinetics. The variance of current fluctuations of such ''noisy'' patches exhibit ed a minimum close to the equilibrium-potential for Cl-. With channels occurring in only 38% of sealed patches and an even lower frequency o f voltage-activated channels, the chloride conductance of the apical m embrane of mitochondria-rich cells did not match quantitatively that p reviously estimated from macroscopic Ussing-chamber experiments. From a qualitative point of view, however, we have succeeded in demonstrati ng the existence of Cl(-)channels in the apical membrane with features comparable to macroscopic predictions, i.e., activation of channel ga ting by cAMP and, in a few patches, also by membrane depolarization.